Mast cells play a pivotal role in the pathogenesis of asthma and other allergic diseases. These reactions are generally initiated by antigen-dependent aggregation of the high affinity IgE receptor (Fc-epsilon-RI) expressed on the cell surface and subsequent release of pro-inflammatory mediators (e.g. histamine, eicosanoids, proteases and cytokines). We wish to identify disease states and clinical populations where hyper-responsive and low-responsive mast cells exist and to identify specific signaling defects responsible for these phenotypes. In addition to the FcepsilonRI, there is an increasing appreciation that other receptors (and other stimuli) may profoundly influence antigen-mediated degranulation. Activating polymorphisms/mutations in, and alternatively spliced forms of receptors and/or signaling proteins may further modulate these responses. Such polymorphisms associated with disease states, for example mastocytosis, may be manifested by exacerbated mast cell-dependent physiology. We wish therefore to also explore the role of other mast cell receptors in disease states and how polymorphisms or alternatively spliced variants of receptors or signaling proteins may produce a hyperactive phenotype. Finally, we wish to explore potential approaches for inhibiting these responses.